SUBJECT: Bending of the pump shaft 1-6

When a centrifugal volute type pump is operating at its best efficiency point (B.E.P.) the bending forces are evenly distributed around the impeller.

If the pump discharge is throttled from this B.E.P. then the fluid velocity is changed and you'll experience an increase in pressure at approximately 240 degrees from the cutwater in the direction of shaft rotation.

It also follows that if the pump capacity increases because of a lack of sufficient head, then this change in flow will cause an increase in pressure in the opposite direction, or at approximately 60 degrees from the cutwater.

The following illustration shows these forces.

 

The formula for the calculation of a multi-diameter shaft looks like this:

How much the shaft bends depends upon the length of the shaft and its diameter. The strength of the shaft has nothing to do with this. The strength only determines if the shaft will break.

The following formula is a simplified version of this longer formula. It is the formula we use for a single stage centrifugal pump with a solid, round shaft and an over hung impeller.

You will note that shaft strength does not enter into the formula. The important number is the Modulus Of Elasticity (E), and as you will learn just about every shaft we use in the pumping business has the same modulus.

Substuting the Moment of Inertia in to the formula gives us:

Since the F is the same in both pumps, along with 3, and 64; and since the Modulus of Elasticity is just about the same for all shaft materials, we can cancel out those terms and we are left with Y = L3 /D4 This ratio then becomes a logical method of comparing two competing pumps that have different shaft sizes. This ratio is often called the stiffness ratio, slenderness ratio, or the flexibility factor.

If we assume that the length of the shaft from the center of the inboard bearing to the center of the impeller is a fixed amount (8 inches or 200 mm.) then we can easily see the affect of varying the shaft diameter in the stuffing box area. The following table shows the relationship:

If you keep this ratio below 60 (2 in the metric system) you will not have too much trouble with shaft bending. If, however, you do not have a low L3/D4 you'll have problems with the shaft packing, mechanical seals and the pump bearings. Keep in mind that we are measuring the shaft diameter. If there is a sleeve on the shaft, do not measure the diameter of the sleeve.

Pump packing has a very poor memory and is not able to follow the bending or deflection of a badly designed shaft. Some packings can be readjusted for the changing leak rate, but in almost every case additional heat will be generated, requiring even more flush water to remove the extra heat. If the packing is not re adjusted with the changing of the pump discharge head then excessive leakage will follow, and along with it&emdash; all of the problems associated with too much leakage.

A bending shaft can be deadly to a mechanical seal because it increases the opportunity for the rotating part of the seal to contact a stationary portion of the pump, causing the lapped seal faces to open and let solids penetrate. If we can keep the lapped seal faces together the seal will not leak and solids cannot penetrate between them. The more shaft movement we have, the more likely the faces are to open.

Bearings are affected by the shaft movement in the same way they are affected by pump/ motor misalignment. There will be an increase in the bearing loading and a corresponding increase in the lubricating oil temperature.

A fourth problem with shaft movement is often overlooked. The pump has several critical tolerances and shaft movement changes them. The most obvious are wear ring clearance, impeller clearance, bearing fit, and seal face loading. These changes can cause additional heat generation, loss of capacity and loss of efficiency.

In summary then, shaft deflection is certainly undesirable. If the L3/D4 is too high you will not be able to operate in slight cavitation or very far from the Best Efficiency Point and in the real world that is not very practical.

Be careful of most small pumps, they often operate at above 3000 rpm and their L3/D4 ratio is atrocious.

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